1. Field of the Invention
The present invention relates to a photocathode which is formed on a member having fine spaces or pores and maintains high sensitivity for a long period of time and a method of manufacturing the same.
2. Description of the Related Art
An example of an electron tube having a photocathode is an X-ray image intensifier. As shown in FIG. 1, this X-ray image intensifier has columnar member 1 consisting of, e.g., a polycrystalline alkali halide for absorbing X-rays 3 and emitting light as a substrate and photocathode (photoelectron conversion layer) 2 formed on this substrate and consisting of a semimetal and an alkaline metal. Reference numerals 4, 5, 6, 7, and 8 represent electron beams, a focusing electrode, an electron lens, an output fluorescent screen, and the X-ray image intensifier, respectively. Substrate 1 converts incident X-rays 3 into visible light, and photocathode 2 emits photoelectrons by a photoelectric effect caused by the visible light. Lens 6 accelerates the photoelectrons and converges them to focus an electron image on screen 7. Screen 7 converts the electron image into a visible image.
The X-ray image intensifier is mainly used for medical diagnosis. Therefore, in order to reduce an X-ray exposure amount of an object to be examined, a demand has arisen for a photocathode of an X-ray image intensifier which has high photocathode sensitivity and can stably maintain the sensitivity for a long period of time.
In order to increase the sensitivity of the photocathode, its composition ratio must be a stoichiometric composition ratio determined by valences of constituent elements or a composition ratio close to it, as described in many articles. For example, in a multi-alkali photocathode consisting of a semimetal Sb (tervalent) and alkaline metals (monovalent) Cs, Na and K, a stoichiometric composition ratio of Sb and a total sum of the alkaline metals is theoretically 1 : 3. If the photocathode has a composition ratio other than the above composition ratio or the composition ratio changes over time, the sensitivity is reduced.
A substrate consisting of a luminescent polycrystalline material such as CsI/Na, Gd.sub.2 O.sub.2 S/Tb, CsI/Tl etc. is formed by a physical deposition method such as vacuum evaporation or sputtering or a chemical deposition method such as CVD. Therefore, in this substrate, unlike in a photocathode of other electron tubes having a substrate of amorphous glass or a metal plate, a large number of grain boundaries, narrow spaces, lattice defect, or pores are inevitably generated. For example, as shown in FIG. 2, when CsI/Na is used, substrate 1 is formed such that light propagates in the longitudinal direction of the columnar polycrystalline of several micrometer-wide CsI/Na and reaches photocathode 2. With this structure, diffusion of the light in the substrate can be reduced, and a large amount of light can be absorbed and incident on the photocathode.
A photocathode consisting of a semimetal such as Sb, Bi, Te etc. and an alkaline metal is formed by, e.g., chemical reaction between the semimetal deposited on a substrate and the alkaline metal effected thereto. However, if narrow spaces or grain boundaries are generated in the substrate as described above, the alkaline metal enters into the narrow spaces, grain boundaries or even crystal itself to change a stoichiometric composition ratio of the photocathode.
For this reason, an interlayer of Al.sub.2 O.sub.3, In.sub.2 O.sub.3, or the like formed by vacuum evaporation is conventionally interposed between the substrate and the photocathode. However, pores or grain boundaries are still generated in the interlayer although they are not so large as those in the substrate, thereby reducing the sensitivity.
FIG. 3 shows results of Auger analysis of a photocathode consisting of a semimetal and a plurality of alkaline metals (Na, K, and Cs) formed on a columnar polycrystal of sodium activated cesium iodide (CsI/Na) through an interlayer of Al.sub.2 O.sub.3. A sputtering time of a rare gas plotted along the abscissa represents a thickness of the photocathode. According to FIG. 3, a composition ratio of Sb and a total sum of the alkaline metals is ranges from 1 : 35 to 1 : 40, i.e., largely differs from the above stoichiometric composition ratio. In addition, the concentration of Cs is significantly high. This is because when a substrate of a polycrystalline member is used, photocathode sensitivity is largely reduced over time. Therefore, in order to compensate for this reduction, the composition ratio is largely shifted from the stoichiometric composition ratio at the cost of sensitivity in an initial stage of use.